The invention is directed to an apparatus for measuring the velocities of gases, especially for the measurement of air for internal combustion engines, consisting essentially of an anemometer of the hot-wire type as a measuring probe placed in the medium to be measured and an electronic unit, wherein the hot-wire anemometer is stablized against drift of the probe characteristics.
In the interest of maintaining the purity of the air it is necessary to reduce the emission of noxious materials from internal combustion engines, particularly from automobiles. This object is, among others, obtained by an improvement of the combustion process. For this it is necessary that there be available the most exact information of the amounts of air taken in, that is with the smallest possible time delay.
This information together with other information such as speed and motor temperature is fed to an electronic control unit which ascertains the necessary amount of fuel for optimum combustion and the time of its addition and is led further to an appropriate regulating unit.
It has already been proposed to employ for this purpose flow measuring apparatuses in the form of orifices or venturi nozzles with appropriate pressure differential measuring instruments. However, in both cases it is difficult to obtain from the slight pressure differential in a simple manner an electrical starting signal to control the fuel.
Furthermore, it is already known to employ measuring instruments for measuring velocity in which an object located in the airstream changes its position. Thereby there is applied an increasing counterforce with an increasing deflection so that the object for any specific velocity is located at a fixed position.
The position of the object can now be scanned optically or mechanically and the results of this scanning changed into an electrical signal. In this process the scanning and above all the long reaction time caused thereby is disadvantageous.
There have also been used devices in which a propeller located in the airstream is scanned optically, mechanically or electrically (magnetically). Also in these cases the moment of inertia of the propeller causes a considerable delay in the reaction time.
Also there are known anemometers of the hot-wire type of different construction in which two temperature dependent resistances, for the most part in the form of two wires, are connected in a bridge together with two temperature independent resistances. One temperature dependent resistance is only exposed to the flow of the gas to be measured and the other only to the temperature of the gas. A disadvantage thereby is that the resistance measuring the temperature of the gas must be shielded from the gas flow. In spite of this the measurement of the flow in this device is still temperature dependent to a certain extent.
To avoid these disadvantages it has already been proposed by one of us (Kolb) to use a hot-wire anemometer which contains two resistance wires of different diameter from the same material with the highest possible temperature coefficients, which together with two temperature independent resistances are connected together to form a bridge and are connected with the input of a difference amplifier whose output is coupled with the bridge input.
The output voltage of this bridge circuit is conducted to an amplifier whose output signal controls the bridge supply current. This apparatus has the advantage that it permits quick and accurate measurement of the velocity of flow of gases independent from the temperature of the gases, whereby the measured result can be easily changed into an electrical output signal.
In Kolb the two resistances of different diameter, for example, are made of wires of platinum, nickel, iron or tungsten, preferably in wound form. The ends of the wires are reinforced advantageously with a good conducting material such as silver or copper. The thin wire preferably has a thickness of 20-150 microns and the thick wire a thickness of 50-250 microns.
FIGS. 1-4 hereof are copies of the drawing Figures of Kolb U.S. patent application Ser. No. 839,383 filed Oct. 4, 1977, now abandoned, and accordingly are marked PRIOR ART.
In FIG. 1, there is shown the measuring apparatus of Kolb. R1 and R2 are the two measuring wires of the probe which are exposed to the airstream to be measured. They are disclosed to have thicknesses of 20-150, and 50-250, respectively, with R2 always being thicker than R1. R3 and R4 form a fixed voltage divider. The bridge diagonal voltage is supplied via the limiting resistors R5 and R6 of a differential amplifier 8. The output of the differential amplifier 8 via the decoupling diode 9 controls the transistor 10. This transistor 10 regulates the bridge supply current which flow for the most part over the resistances R1 and R2 and only a very small part flows over the substantially high ohmic resistances of the voltage divider R3 and R4. The Zener diode 11 insures that the inverting and non-inverting input of the voltages of the differential amplifier 8 do not go below a fixed minimum height. At resistance R5 of the current flowing through the measuring bridge produces a dop in voltage which can be provided at terminals 12 and 13 as output voltages. The operational voltage is supplied between terminals 14 and 13.
In FIGS. 2, 3 and 4, R1 is the thinner measuring wire and R2 the thicker measuring wire, both of which with their reinforced ends 21 are mechanically connected with the electrically conducting parts 22, 23 and 24 of the carrier body 26. In FIG. 2 the resistances R1 and R2 are arranged parallel, in FIG. 3 in V form and in FIG. 4 in Y form whereby the low part 27 of the Y is formed to R1 and R2 through the necessary supply leads. The connection to the electronic unit takes place via feed lines 25.
The operation of the Kolb device can be explained as follows:
The voltage divider R3 and R4 is so dimensioned that at a flow velocity of =0 a feed circuit flows through the resistance R1 and R2. The resistance R1 as a result is more strongly heated up than the thicker wire consisting of resistance R2. The feed current adjusts itself automatically so that the bridge is in equilibrium, i.e., R1:R2=R3:R4. At R5 as a result there is found a specific drop in voltage.
If now the resistances R1 and R2 are impacted with flowing air, the hotter resistance R1 cools more quickly than the colder resistance R2, the bridge falls out of equilibrium and the differential amplifier 8 produces a higher output current which controls the transistor 10 to increase the current which is flowing into the bridge such that the ratio R1:R2-R3:R4 is again produced. At R5 there is now a higher voltage drop.
According to Kolb, temperature changes of the air measured have no influence on the measuring result since both resistances R1 and R2 have the same temperature coefficient and the bridge equilibrium is not disturbed thereby.
According to Kolb, his device exhibits the particular advantage that the output voltage (terminals 12 and 13) within very wide limits is independent of the applied operational voltage between the terminals 14 and 13. This device according to Kolb holds the current necessary for the bridge equilibrium automatically constant, independent of the at times applied operating voltage so long as the applied supply voltage is sufficiently great to cover the voltage drop at the resistance R5, at the diode 11, the resistances R1 and R2 and the lowest voltage drop at the transistor 10.
At high air flow velocities a very strong current flows through the resistance R1. If the resistance R1 is extended as a simple wire section over the entire cross-section of the air flow, there can be in the section of the wire nearest the wall an overheating of the resistance R1 at such places caused by the much slower or almost non-moving air layers. To avoid this it is advantageous according to Kolb to reinforce the ends of both resistances R1 and R2 in the area of the slower flowing air layers near the wall and thus to substantially reduce their resistance and their effectiveness in this area, or to entirely eliminate it. This precaution, Kolb discloses, has the additional advantage that the dirty course of flow of the air layers near the wall does not influence the measurement.
However, in practice it has proven that the characteristic of this anemometer probe is not stable, but in the course of time shows deviations from the calibrated characteristic. This drift above all is traced to the deposition of dust on the wire surfaces which changes the heat transmission and additionally is traced to erosion and deformation of the surfaces of the measuring wire by rebounding dust particles contained in the gas stream.
Besides such a anemometer of the hot-wire type has the property of detecting the amount of the velocity of flow independent of the direction of flow. In many cases of use, e.g. even in the intake system of internal combustion engines, the velocity of flow is interfered with by temporary fluctuations. These fluctuations can be so great that they can temporarily reverse the direction of flow. In such cases such an anemometer of the hot-wire type shows a false average value.